Bullet seals are described in detail in U.S. Pat. No. 5,611,547 where seals 80 and 82 are disposed in mirror image with an o-ring 78 between them to handle pressure differential in opposed directions. These seals have a generally pointed end and an opposite finned end adjacent the o-ring. These seals were designed to replace the stacks of chevron shaped seals shown in FIG. 1 of that patent. There were no backup seals used for pressure differentials in a given direction and no efforts to actuate the finned end beyond the axial pressure from fluids in the string. Other related seal designs are shown in U.S. Pat Nos. 5,879,010; 7,341,258; 5,524,905; 7,363,981; 6,318,729; 7,819,184 and 7,401,788.
Of the bullet seals that have been deployed in the past there have been issues with reliable performance particularly where significant temperature fluctuations of over 200 degrees F. could occur. These seal assemblies had no backup as illustrated in FIG. 1 with opposed bullet seals 10 and 12 separated by an o-ring 14 and disposed in a mirror image layout for opposing pressure differentials from either an uphole or a downhole direction. The past design focused on ease of assembly and a need to optimize a sealing system in a small confined space. The bullets were installed in an activated configuration. In some applications backup bullet seal pairs were used where the primary and the backup seals were identically configured and installed in a configuration that application of a net differential pressure in one direction activated both the primary and secondary seals due to axial compression through the o-ring in between opposed seal pairs and more rigid end rings adjacent the tapered ends of the seals.
What is needed and provided by the present invention is an assembly of bullet seals with a backup that is delayed in being deployed using primarily a temperature effect on a material that prevents advancement of a ring between end fins. The backup seal is not actuated until pressure is communicated to it such as by a failure of a primary seal and then there is the effect of temperature of well fluids to allow the t-shaped ring to advance as the material on the leading branch of the t-shaped ring softens to allow pressure differential to advance the ring and spread the end fins of the bullet seal to enhance its performance. These and other features of the present invention will be more readily understood by those skilled in the art from a review of the detailed description of the invention and the associated drawings while recognizing that the full scope of the invention will be determined by the appended claims.